The present invention relates generally to wireless communication devices and in particular to a wideband envelope tracking amplification system of a wireless communication device.
Power amplifiers for wireless transmission applications, such as radio frequency (RF) power amplifiers, are utilized in a wide variety of communications and other electronic applications. Ideally, the input-output transfer function of a power amplifier should be linear, that is, should implement a constant gain adjustment and phase adjustment of an input signal, wherein a perfect replica of the input signal, increased in amplitude, appears at the output of the power amplifier.
In addition, for greater efficiency, various RF systems, such as cellular systems, attempt to run power amplifiers at or near their saturation levels, in which the actual output power of the amplifier is just below its maximum rated power output level. This power output level is generally related to the supply voltage (or supply power) to the power amplifier, such that a greater supply voltage will produce a correspondingly greater output power from the amplifier; for higher power input signals, a correspondingly greater actual power output is required to maintain the amplifier at or near saturation. In various prior art amplifiers, however, the supply voltage to the power amplifier is fixed. Given a typical usage situation in which actual power output from the amplifier may vary by a range of several orders of magnitude, use of a fixed supply voltage is highly inefficient, as output power is often an order of magnitude or more below its maximum, and the power amplifier is not maintained at or near its saturation levels.
Various techniques have evolved to vary the supply voltage to maintain the power amplifier at or near saturation. One such technique is power supply modulation (PSM) that varies, or modulates, the supply voltage to the power amplifier in order to maintain the power amplifier at or near saturation while the input signal varies over time. For PSM, the supply voltage of the amplifier tracks the input signal variations, typically utilizing a signal detector in conjunction with a tracking power supply. In the prior art, however, the various PSM techniques have generally been limited to narrowband applications, or have poor efficiency characteristics.
For example, one prior art PSM technique, known as xe2x80x9cenvelope elimination and restorationxe2x80x9d (EER), utilizes a limiter to provide an essentially constant drive level to the power amplifier to maintain the amplifier in a hard saturation state and increase efficiency. Use of the limiter, however, greatly expands the bandwidth of the RF signal input to the amplifier and requires very accurate tracking of the input signal envelope, with a power supply switching frequency approximately ten times greater than the bandwidth of the RF input signal. As these switching frequencies increase, the transistors within the tracking power supply become less efficient, resulting in excessive power losses. The resulting bandwidth expansion of the limiter also requires the bandwidth capability of the amplifier to be significantly greater than the input signal bandwidth, limiting the EER configuration to narrow bandwidth applications, such as amplitude modulation (AM) RF broadcasts.
Another prior art PSM technique, known as xe2x80x9cenvelope tracking,xe2x80x9d does not utilize the limiter of EER. Consequently, envelope tracking power amplification systems may be more suitable for higher bandwidth applications. FIG. 1 is a block diagram of an exemplary envelope tracking power amplification system 100. A radio frequency (RF) signal 101 is coupled to an input 102 of amplification system 100. A signal coupler 104 samples input signal 101 to produce a sampled input signal 105 and routes sampled input signal 105 to an envelope tracking power supply (ETPS) 106. ETPS 106 tracks or detects an envelope of sampled input signal 105 to produce an envelope detector signal, typically a voltage, and produces a variable supply voltage 107 based on the detected envelope of input signal 101. ETPS typically includes a switching power supply whose switching pulse width or frequency is varied in order to track the envelope of input signal 101 and produce variable supply voltage 107.
ETPS 106 sources variable supply voltage 107 to an RF power amplifier 108. Variable supply voltage 107 is designed to maintain RF power amplifier 108 at or near saturation and to increase the efficiency of power amplification system 100 over a wide range of variation in input signal 101. When input signal 101 is a wideband RF signal, the switching power supply of ETPS 106 must have a very rapid response in order to track RF input signal 101. However, if variable supply voltage 107 is to accurately reproduce the envelope of RF input signal 101, then the switching frequency of ETPS 106 should be 5-10 times the bandwidth of input signal 101. For example, if input signal 101 has a bandwidth of 20 MHz, as is common in multi-carrier amplification systems, then ETPS 106 should have a prohibitively high switching frequency of 100-200 MHz.
In order to resolve the requirement for a prohibitively high switching frequency power supply, schemes have been proposed for utilizing multiple voltage supplies in implementing the ETPS, such as in U.S. Pat. No. 5,239,275, entitled xe2x80x9cAmplitude Modulator Circuit Having Multiple Power Supplies,xe2x80x9d and U.S. Pat. No. 5,736,906, entitled xe2x80x9cPower Supply Modulator Circuit for Transmitter.xe2x80x9d Such schemes typically involve selecting a voltage supply of the multiple voltage supplies, or serially connecting one or more voltage supplies of the multiple voltage supplies, based on a detected instantaneous magnitude, or amplitude, of the input signal. However, multi-carrier input signals typically have a wide dynamic range, often in the range of 10-20 dB, due to high short-term peak-to-average power ratios caused by the multiple carriers and due to long term fluctuations in average power due to variations in traffic loading. In order to track a multi-carrier input signal, the ETPS may require a subdivision of an input signal voltage range into as many as 10 to 12 input signal amplitude steps and correspondingly may require as many as 10-12 voltage supplies. The use of such a large number of voltage supplies is both prohibitively expensive and complex.
Therefore, there is a need for a high efficiency, low cost method and apparatus for tracking a wideband RF signal under high dynamic range conditions.